In modem digital processes for the reproduction of color documents, pictures and packages, desktop publishing and color management have become progressively more important. In desktop publishing, the designer these days generates on his computer in electronic form not only the graphic concept and the color design, but specifies progressively more of the reproduction technological parameters concerning the final production and multiplication of the document, such as, for example, the printing process, the paper quality or special colors. Since test prints are expensive and time consuming, it is desired for this application that the designer can simulate and test the color reproduction properties of his electronic design according to a special printing process.
The communication and reproduction of consistent colors is made possible by the use of color management technologies. In one color management system (CMS), the input and output systems involved in the workflow are calibrated and characterized with respect to color measurements with the use of color measurement apparatus. The connection between absolute measured color values and system specific color control signals is stored in so called apparatus profiles. Color information can be communicated and reproduced across different media and systems by the combination of device profiles.
The construction of existing color management systems has been standardized and described by ICC (International Color Council; see, for example http://www.imaging.org for the newest version of the ICC standard) and is summarized in the publication of the company Logo GmbH, a corporation of the GretagMacbeth Group in August 1999 under the title “Post Scriptum on Color Management, Philosophy and Technology of Color Management”, by the authors Stefan Brηes, Liane May and Dietmar Fuchs.
By using color management and device profiles, the designer has the opportunity to visualize on his screen the color reproduction of different printing processes, or to simulate it with a digital printer. However, the options and the precision of such digital test methods are still limited at this point in time. Material or process specific parameters which influence the visual color perception, but can be captured not at all or only badly with the actually used color measurement devices generate difficulties for a correct visualization. Typical examples are substrate materials with differing surface properties, optical brighteners, or fluorescent colors.
Samples of the same color but with different surface properties have different appearances. A sample with a glossy surface is perceived darker and of more saturated color than one with a diffusely reflecting surface.
The effect of optical brighteners and fluorescent colors strongly depends on the illumination spectrum. For a correct evaluation, the light source in the measurement apparatus must be able to be adapted to real observation conditions.
So called effect color layers based on metallic or pearlescent pigments are of increasing importance for packaging and advertising. They are distinguished by strongly illumination or observation angle dependent reflection properties. A description of effect color layers is found, for example, in the articles “Pigments—Coloring Agents and Functional Carriers” of C.D. Eschenbach and “Pearlescent Pigments” of G. Pfaff in Spektrum Der Wissenschaft, October 1997, pgs. 94-99 and pgs. 99-104. Such angle dependent special effects can at this point in time not be captured within a color management system.
A strongly angle-dependent surface gloss also occurs with printed metallically glossy substrate materials or foils, which must be captured by measurement technology for a correct reproduction.
For the application as a built in sensor, a measurement system must fulfill the following basic requirements: it must permit fast measurement cycles, have a compact construction and low weight, must be robust and have a long service period, and must be as maintenance free as possible. Furthermore, a measurement system must enable a contact free measurement geometry to the sample in order to be used in online or inline operation in an automated system. For color management applications in the graphics area, color measurement devices with 45E/0E or 0E/45E measurement geometry are currently used, which are realized in compact construction. A typical example is the spectrophotometer “Spectrolino” of the company Gretag-Macbeth AG and the reference EP-A 1067369. The measurement technology used is based on an incandescent lamp as illumination source and a diode array—grating—spectrometer for the spectral evaluation in the measurement channel. The “Spectrolino” spectrophotometer can be added as a measurement head onto an xy—displacement table, for example, the “SpectroScan” type of the company GretagMacbeth AG, which allows for the automated measurement of a large number of color samples possible for the generation of a printer device profile.
The measurement technology used in the “Spectrolino” is not suitable for use as a built-in sensor for various reasons. The incandescent lamp has a finite service life. The measurement system must therefore be designed especially for an easy lamp replacement. The filament of the incandescent lamp is sensitive to vibrations, which creates difficulties upon installation into motorized systems. The measurement system is also sensitive to variations in the distance between the sample and the measurement optics. As a consequence, the measurement in these apparatus is carried out in direct mechanical contact with the sample.
The use of a color measurement device with 45E/0E measurement geometry in digital printing systems for color management applications is described, for example, in DE-C 197 22073.
Angle dependent reflection properties cannot be captured with the 45E/0E geometry. Known processes of this geometry can include the use of additional optical filters which can be moved into the elimination and measurement channel for the evaluation of material parameters, such as surface effect and fluorescence excitation of optical brighteners. For the elimination of the surface effect, a measurement with crossed linear polarization filters can be used and light type conversion filters can be used for the fluorescence excitation. For automatic measurement systems, this functionality must be achieved with mechanically operated components, for example, with a filter wheel as realized in the spectrophotometer “Spectral Eye” of the company Gretag-Macbeth AG. Moveable mechanical components are not suitable for compact sensor systems.
Alternative technologies in combined measurement devices for color and surface properties are based on a diffuse measurement geometry with an Ulbricht Sphere or the use of a gloss measurement.
A gloss trap can be incorporated in the Ulbricht Sphere in the diffuse measurement geometry, which eliminates oriented light reflected from the probe surface in the measurement channel. An advantageous embodiment of this measurement principle is described, for example, in EP-A 0964244. However, the diffuse measurement geometry must by definition be realized in direct contact with the sample to be measured. The measurement geometry with the Ulbricht Sphere is not suited for miniaturization and the manufacture of a well reflecting Ulbricht Sphere is costly. Furthermore, the geometry of the Ulbricht Sphere is not directly applicable for the fluorescence measurement and must be especially calibrated.
In gloss measurement devices, the sample is directionally illuminated with a narrow band light source and the portion of the light directionally reflected by the surface is measured with a detector. A degree of gloss which is characteristic for the surface is determined from the measured value. This degree of gloss represents the surface quality, but not directly the visual color perception and is therefore not suited for visualization applications in the color management field. Combined color measurement devices with 45E/0E measurement geometry and integrated gloss measurement are described in U.S. Pat. No. 5,377,000 and U.S. Pat. No. 4,886,355. A commercial product with this functionality is available from the company BYK-Gardener under the name “Color—Guide Gloss”.
Different manufacturers offer multi-angle spectrophotometers for color measurement technology. Typical examples herefor are the apparatus with product designation “MA68IL” available from the company X-rite, Inc. (Grandville, Mich., USA), product designation “CM512m3” available from the company Minolta Co., Ltd. (Osaka, Japan), as well as product designations “CE640” and “CE740” available from the company GretagMacbeth AG (Regensdorf, Switzerland). Although some of these apparatus are portable, they have a complex mechanical construction and must be mechanically placed onto the sample for measurement. These systems also cannot be used as built-in sensors because of their size.